The elephant hawk-moth has declined by 11% over the last 50 years, according to Rothamsted Insect Survey data.
Photograph: Callum Macgregor

Moths are declining in abundance by 10% each decade in Britain but the average weight of moths caught in traps is still double what it was in 1967, according to a new study.

Researchers studying the biomass of moths caught in the world’s longest-running insect survey said their findings suggested that if there had been an “insect armageddon” in Britain, it had occurred before scientific recording began in 1967.

Rather than a precipitous recent decline, scientists at the University of York found that moth populations had boomed following the 1976 heatwave – and moth biomass today was still twice as high as 1967 levels – but there had been a steady decline in abundance since 1982.

“It’s absolutely not the case that everything is fine,” said Dr Callum Macgregor. “We do know that insects are in long-term decline as a whole, and also that the majority of insect species are declining.

“The concerning thing about the decline is that it’s over a 35-year period and there’s no real sign that that long-term declining trend is reversing.

“Having said that, the implication of a phrase like ‘insect Armageddon’ is that it’s an end-of-days scenario and it’s almost hopeless, and I don’t think it is hopeless. There is still time and opportunity for us to turn things around and make positive changes in the way we use our land.”

Biomass is an important measure because it signals the quantity of moths available to pollinate plants and provide food for birds, bats and small mammals. Researchers believe that moth declines are likely to mirror falls in the populations of other flying insects.

Two common causes given for the decline of moths are agricultural intensification and light pollution in urban areas.

The study found that traps in urban areas and on arable land had caught a far lower abundance of moths than grassland and woodland.

Between 1983 and 2017, the average weight of moths caught in a woodland trap each year was 81,889 milligrams. In a grassland trap it was 44,828mg and in traps based on arable farmland the weight of moths fell to 36,280mg. In urban areas – where moths are affected by light pollution – it was 31,673mg.

But while moths in grassland have declined by 18% since 1983 and in woodland by 15%, moth abundance remained stable on arable land.

According to Macgregor, this could show that chemical-sensitive species had already been obliterated from farmland by agricultural chemicals used in the 1960s before the Rothamsted survey began.

“A lot of our farmland was pretty intensively farmed already by 1967 so species that were going to be driven out by agriculture might have already gone by the time of the beginning of this dataset,” said Macgregor. “If we went back to 1930 or 1850 there could be a massive decline but there’s not any way to know that unfortunately.”

The study found that rather than a precipitous recent decline, there has been a slower, steady decline since 1982 and considerable fluctuation between years. The researchers believe these fluctuations are caused by extreme weather and climatic changes.

It appears that the dry weather in 1975 and heatwave in 1976 caused a massive boom in British insect populations, with the average moth trap biomass increasing by 207% between 1967 and 1982. In the mid-1990s, there was a crash with the average biomass per trap of 93,100mg in 1996 falling to 70,486mg in 1997 and 45,773mg in 1998 before stabilising at around 50,000mg at the turn of the century.

“We found that short-term studies and infrequent sampling can give erroneous estimates of biomass change,” he said. “The complexity of insect population change requires more, better and longer-running data if we are to draw robust conclusions, particularly for parts of the world where insect data are limited.”

Matt Shardlow, chief executive of Buglife, said: “This research provides further evidence that there is a current long-term decline in insect biodiversity. The historical sharp increase in the estimate of moth biomass during the 1970s on these 34 sites is new and unexpected.” He added: “There is a need for additional corroboration and explanation.”